The present invention relates generally to the ability to improve the surface smoothness on composite materials for use in the construction industry. More particularly, the present invention relates to the use of a water spray to impart energy to the surface of the material when the composite material is still in a slurry or semi-slurry state where such energy acts to break up flocs or clumps of composite materials that render a rougher surface on the finished product.
The United States Gypsum Company""s gypsum fiberboard process, as shown and described in U.S. Pat. No. 5,320,677, and herein incorporated by reference in its entirety, describes a composite product and a process for producing a composite material in which a dilute slurry of gypsum particles and cellulosic fibers are heated under pressure to convert the gypsum, i.e. calcium sulfate in the stable dihydrate state (CaSO4.2H2O), to calcium sulfate alpha hemihydrate having acicular crystals. The cellulosic fibers have pores or voids on the surface and the alpha hemihydrate crystals form within, on and around the voids and pores of the cellulosic fibers. The heated slurry is then dewatered to form a mat, preferably using equipment similar to paper making equipment, and before the slurry cools enough to rehydrate the hemihydrate to gypsum, the mat is pressed into a board of the desired configuration. The pressed mat is cooled and the hemihydrate rehydrates to gypsum to form a dimensionally stable, strong and useful building board. The board is thereafter trimmed and dried.
One of the many advantages of the process disclosed in the ""677 patent is that the surface of the resulting gypsum panel can be smoothed, or in the alternative, textured as the panel is being formed. The challenge in treating the surface of gypsum fiberboard during in-line processing is the timing of the treatment made on the slurry or wet mat. The smoothing as taught in this application begins while the material is still in a slurry or just beginning to form a semi-slurry state.
As the rehydratable calcium sulfate hemihydrate and cellulosic fibers in a slurry form leave the head box, and are disposed upon the conveyor belt or forming wire, the slurry will have a temperature generally in the range of about 200xc2x0 F. +/xe2x88x9210xc2x0. Thereafter, as the slurry is spread to create a forming pond across the conveyor the action of vacuum pumps begins removal of the free water and the temperature drops significantly and the rehydration process begins.
As the slurry exits the head box, the dewatering process begins with the action of the vacuum pumps. However, commingled crystals and fibers may collect and form clumps or flocs, which is undesirable at the surface of the product. It is preferred that the clumps or flocs have a greatest dimension less than approximately 6 mm. When rehydrated, clumps of commingled crystals and fibers larger than 6 mm impart an undesirable roughness to the surface of the finished material. Roughness of wet felted products is detrimental to final installations where surface finish is important to final applications, such as painted surfaces (walls) and thin overlays (vinyl laminations). One contributor to such roughness is the condition present in the forming pond during substrate manufacture.
Typically at least two factors increase roughness: high consistencies and long fiber content. These are known to be minimized by the addition of water to lower consistencies or by agitation in the slurry pond. Both of these methods though have other undesirable effects. The addition of water adversely affects drainage rates and may cause line speed decreases and increase vacuum demands. Agitation in the pond may adversely affect the preferred form of matrix formation which is collective sedimentation, as the slurry is dewatered into a filter cake if the agitation is applied in the wrong stage of formation of the product or at the wrong level in the pond. In addition, if the agitation is used with a slurry having a raw material mix of widely divergent densities or settling rates, such as is common with high filler fine paper or wet process gypsum fiberboard, the lower density material will separate from the higher density material, causing a nonuniform product in the case of the wet process gypsum fiberboard. The separation of material may also result in a decreasing first pass retention on the forming wire as higher density materials that should remain on the forming table are drawn off by the vacuum boxes during dewatering and returned upstream of the head box for recirculation into the slurry.
A variety of other methods have been attempted to modify forming pond characteristics to improve surface smoothness, such as vibrating rods, vibrating plates, rotating rolls, smooth top plates, water sprays, etc. The use of the water spray described herein, applied in the area of the wet line, has yielded better surface smoothness when compared to fiberboards smoothed using the other methods.
The present invention relates generally to producing gypsum fiberboard panels with a smooth surface texture. More particularly, the present invention relates to the use of a water spray to impart a smooth surface texture to gypsum fiberboard panels.
A water spray with proper pressure, angle of incidence and distance from surface, may be applied in the formation pond of wet felted products, particularly those formed at high consistency, to modify slurry properties during formation and improve properties, in particular surface smoothness of the final panel.
With reference to the process of the ""677 patent, in the present invention energy is selectively applied to the top of the forming pond close to the wet line by a water spray of sufficient energy to disperse clumps in the pond surface and slightly below it, but insufficient energy to disrupt the pond more than slightly below the surface. This energy disperses flocs or clumps of commingled material during mat formation without disrupting the preferred method of collective sedimentation.
The water spray extends across the forming pond between the opposing side dams of the forming table. The water spray is applied perpendicular to the surface of the slurry. It has been found that a spray applied at a low flow rate or low pressure 10 psi to 50 psi can impart the required energy with the advantage of reduced water addition and subsequent required removal over that of a conventional water spray designed to lower consistencies.
One method of generating this water spray is to connect a source of water to a distribution manifold equipped with a plurality of nozzles. The water source may be any water source that can deliver water at a pressure, such as a gravity tank, municipal water supply, or hydraulic pump. The nozzles are installed to deliver a uniform spray along a two dimensional path. Experimentation with various pressures and flow rates has shown superior performance with decreasing pressures or decreasing flow rates. The performance of the water spray method described herein is dependent upon the imparting of energy by the water spray, not upon the simple addition of water.